The present invention relates to a suction/blowing mold system. More particularly this invention concerns the mold for such a system.
In a standard suction/blowing mold system for producing complex three-dimensional hollow articles, typically of plastic, a hollow parison or preform having a closed leading end is Reproduced by an extruding apparatus. This leading end is engaged in an upper intake end of a mold passage formed between two fitted-together female mold halves and of somewhat greater cross-sectional size than the preform. Suction is applied to the opposite lower or output end of the mold passage to draw the preform while it is still warm and soft down through the mold as it is either emitted by the extruding apparatus or advanced by a manipulator.
Once the preform is threaded completely through the mold cavity, its leading and trailing ends are pinched closed at the upper and lower passage ends. A needle is then inserted centrally into the preform and gas is blown into it to expand it to fill the mold cavity. Once the material of the preform has cured, the mold is opened and the finished workpiece is removed. It will have the exact shape of the mold passage with no flashing or sprues. In particular its inner surface will be perfectly smooth and without seams. Such a system, as described in xe2x80x9cPlast-verabeiterxe2x80x9d (vol. 44, no. 12, 1993, pages 46-49) or in xe2x80x9cTech-nische Mitteilungen Kruppxe2x80x9d (2/1998/ pages 49/50), is used, for go instance, to make complexly shaped manifold or feed tubes for appliances.
U.S. Pat. No. 4,878,828 of Wollschlager describes a mold-closing device which is intended to hold closed such a mold against the modest outwardly directed forces that tend to spread the die halves during inflation of the parison. This equipment is much simpler and less expensive than the complex mold closing and clamping systems used with higher-pressure molding operations.
German 198 45 269 of Koetke describes a system where the parison is fed through a short tubular fitting to the inlet end of the mold passage and warm air is sucked through the passage so as to draw in the parison and feed it even through a relatively complexly shaped passage. The air flow around the parison pulls it along while keeping it largely out of contact with the inner surfaces of the passage, making it possible to mold very complex shapes. The considerable suction here is a problem in some instances as lateral leaks can cause local air currents that damage the parison, for instance by cooling a portion of it so that it later cannot be deformed to the desired shape.
It is therefore an object of the present invention to provide an improved suction/blowing mold.
Another object is the provision of such an improved suction/blowing mold which overcomes the above-given disadvantages, that is which allows even very complexly shaped and small-diameter objects to be made in a fairly simple manner.
A molding apparatus has according to the invention a pair of mold halves having faces engageable together to form a nonstraight passage having an intake end and an output end, extruder means for forming a hollow plastic preform and feeding it to the intake end of the passage, pump means connected to the output end of the passage for drawing gas therefrom and thereby applying drawing the preform through the passage, and a seal carried on one of the faces, extending along the passage, and engaging the other of the faces for preventing gas flow between the faces into the passage when the mold halves are closed together.
Thus with this system no air will be sucked into the passage between the mold halves which, as described above, are not held together with substantial force. This is particularly useful in a suction/blowing molding system where the passage is often of fairly complex shape, so that ensuring that the two mold halves seat sealingly against each other is quite difficult.
The system of this invention allows even relatively small-diameter or slim preforms to be sucked through the mold passage. Since no cold ambient air will be leaking in from outside as a result of the suction applied to the outlet end of the passage, the preform will not be locally cooled so that it will remain plastic and will be easily deformed to the desired shape.
According to the invention the seal is fixed to the one face. More particularly the one face is formed with a groove holding the seal. This groove can be undercut, typically being of isosceles-trapezoidal section.
To further facilitate the manufacture of slim and/or complexly shaped workpieces means is provided for introducing heated gas into the intake end around the preform. A controller maintains a temperature of the heated gas generally constant.